Certain types of applications require a load to be driven with an electrical burst excitation signal of relatively high voltage but brief duration. For example, ultrasonic instruments of the kind used for medical diagnosis often employ an array of piezoelectric transducers. Each transducer represents a capacitive load and must be suitably driven to make its contribution in the generation of a wavefront of ultrasonic energy which is applied to the target under observation, e.g. a portion of the human body. An echo waveform in the form of reflected energy is subsequently received by the transducers. Each transducer must then convert the reflected energy portion received by it back into an electrical signal for subsequent analysis. By phasing the transducer array, the emitted energy is focussed to provide a beam capable of scanning the sector of interest. Steering of the beam to the target may be accomplished by suitably delaying the excitation of respective transducers, while the energy transmitted may be controlled by varying the amplitude of the excitation.
In such an arrangement each transducer typically has its own excitation circuit, which drives the transducer during successive time intervals that alternate with the periods during which the transducer receives the reflected energy. The circuit responds to an input trigger signal which must be compatible with the operating frequency of the transducer. For one type of transducer which has been used for ultrasonic medical applications, the operating frequency range may extend from 2 to 5.times.10.sup.6 Hz.
In prior art devices, such as the P100A Series Ultrasonic Pulse Generator made by Metrotek, Inc., excitation is provided by applying a single pulse spike to the transducer. Typically this technique is implemented by firing an SCR to dump the stored charge of a capacitor into the transducer. A major disadvantage of this technique is that only a limited amount of energy can be transferred by means of a single pulse spike. Hence the excitation of the transducer, and consequently the magnitude of the echo reflected from the target, are limited. For example, at a pulse frequency of 3.5 MHz, the pulse width is fixed at approximately 150 nanoseconds and the energy content is thus determined for a given pulse amplitude. While the pulse amplitude may be varied, the full voltage produced in such a circuit divides between the impedance of the capacitor and that of the transducer itself. Therefore the amplitude of the pulse applied to the transducer is reduced and the energy transferred to it is necessarily limited. Furthermore, since the applied pulse amplitude is dependent on the transducer capacitance, manufacturing tolerances associated with the transducer will cause the applied pulse to have a non-controllable variation with different transducers.
Another difficulty arises where transducers are to be driven that operate at different center frequencies. Because of the different operating frequencies, the characteristics of the respective transducers will typically be different. Each transducer will therefore require a unique excitation circuit, one having parameters that are different from those of the driver circuits of the other transducers. Thus the flexibility of interchanging transducers without also interchanging the excitation circuits is lacking and the cost of the overall system is increased.
Since the storage capacitor in prior art driver circuits charges at its own rate before it can be discharged by firing the SCR, the excitation of the transducer cannot be carried out at a rate equal to the transducer center frequency. The excitation of the transducer with a multiple cycle burst at the transducer center frequency, where such multiple cycle burst is phase coherent with another signal, such a system clock signal, is therefore precluded. This limitation presents a distinct disadvantage and it virtually eliminates the possibility of programmed control, i.e. the selective variation of the energy transferred to the transducer by means other than supply voltage variation.